1. Field of the Invention
The present invention relates to a piston for an automatic transmission (hereinafter, refer to as “BPS”) used for coupling a clutch in the automatic transmission of a motor vehicle. More particularly, the present invention relates to a structure of an inner peripheral seal portion arranged in a cancel plate of the BPS.
2. Description of the Conventional Art
Conventionally, a BPS 100 used for coupling a clutch in an automatic transmission such as an AT, a CVT and the like for a motor vehicle is provided with a piston member 110 moving in an axial direction in an inner portion of a closed-end cylindrical cylinder 101 and defining a compression chamber M together with the cylinder 101, a cancel plate 120 opposing to a bottom portion 102 of the cylinder 101 beyond the piston member 110 and defining a balance oil chamber N together with the piston member 110 and the cylinder 101, and a return spring 130 functioning such as to push back the piston member 110 which is pushed down by the compression chamber M being pressurized, as shown in FIG. 3, and only an outer peripheral seal portion 121 is arranged at an outer peripheral side of the cancel plate 120 for retaining a centrifugal hydraulic pressure.
In other words, since the cancel plate 120 is provided with only the outer peripheral seal portion 121, and is not sealed at an inner peripheral side, the centrifugal hydraulic pressure of the balance oil chamber N becomes small in comparison with the centrifugal hydraulic pressure of the compression chamber M, and it is impossible to effectively cancel the centrifugal hydraulic pressure of the compression chamber M at a time of cancelling the coupling of the clutch. Therefore, as shown in FIG. 4, there has been structured such as to increase the centrifugal hydraulic pressure of the balance oil chamber N by arranging an inner peripheral seal portion 123 constituted by a seal lip 122 which is in close contact with the inner peripheral tube portion 103 of the cylinder 101, on an end portion 124 at an inner peripheral side of the cancel plate 120, so as to secure a sealing performance of the balance oil chamber and improve a shift response.
However, since it is necessary for the inner peripheral seal portion 123 arranged on the inner peripheral side end portion 124 to absorb a play in a diametrical direction of the cancel plate 120 by the seal lip 122, there is a risk that an abrasion of the seal lip 122 is promoted so that the hydraulic pressure cannot be retained, in the case that a small amplitude oscillation is generated on the basis of a pressure fluctuation. Further, in the case that an inclination or the like is generated by eccentric attachment of the cancel plate 120 or a biased load application of the return spring 130, the seal lip 122 is pressed in one direction. As a result, a biased abrasion or a lack of an eccentricity following characteristic is generated in the seal lip 122, and there is a risk that the hydraulic pressure cannot be retained by the inner peripheral seal portion 123.
In this connection, Japanese Unexamined Patent Publication No. 2004-060750 discloses a structure which prevents an operating fluid from leaking and prevents an abrasion powder from making an intrusion by installing a seal member of a rubber-like elastic material for sealing a fitting surface between a cancel plate and an inner peripheral tube portion of a housing to an inner peripheral portion of the cancel plate while compressing the seal member in a state of having a predetermined fastening margin, for the purpose of shortening an axial dimension so as to downsize a clutch apparatus. However, the disclosed invention has a different object from an object of the present invention.